The present invention relates to methods of and apparatus for displaying electronically stored information in a human readable form.
Flat panel displays are now part of everyday life. The best known type is the liquid crystal display (LCD) which is used widely in application varying from watch displays to large screen television displays. The technology is applied in household products, the automotive and aerospace industries, medical products and professional products. The above list is not exhaustive, but indicates how widely the technology is used.
Despite the wide range of applications of LCDs, they are not perfect for all applications. Indeed, cathode ray tubes (CRTs) are still very widely used. Other flat panel display technologies exist, although none has the significance of LCDs.
One shortcoming of conventional LCD technology is that it cannot be viewed at a high angle of incidence. In fact many LCDs are not readable at as little as only 20 degrees off-axis. This is largely due to the fact that LCDs utilise an electro-optic effect which occurs through the depth of the display. CRTs in contrast operate by causing a phosphor coating to emit light when it is bombarded by electrons. This is a surface effect which can be seen from a wide range of angles of incidence.
A different type of display technology which has not been widely adopted is the electrophoretic display (see for example Ota et. al. Proc. IEEE, July 1973, pp. 832-836). The basic principle exploited in an electrophoretic display (see FIG. 1) is that when charged particles are placed in an electric field they will migrate along the field lines. If relatively light coloured particles are suspended in a dark liquid between transparent electrodes, then the electrodes will take the colour of the particles when they are driven towards the electrodes. By segmenting the electrodes, a pattern of the particles can be selectively laid down to form a display.
When the display is viewed, the viewer sees the scattered particles. The effect is very similar to that of viewing paper as there is no change in contrast with viewing angle. This is a significant advantage over the LCD. However, owing to the fact that the display operates by particles migrating across the cell, the speed of operation is not as fast as that of a liquid crystal display.
A second difficulty with electrophoretic display technology, and one which has limited the exploitation of this technology to date, is the fact that there is no threshold for the electrophoretic effect, in other words, whatever electric field is applied to the cell the particles move. This means that a simple row and column matrix addressing system cannot be implemented. The virtues of a matrix addressing scheme have been described by several authors, for example Alt and Pleshko (IEEE Transactions on Electron Devices, Vol ED-21, No. 2 February 1974 pp146-155). The benefit of a matrix addressing scheme is that a 2 dimensional array consisting of N rows and M columns (i.e. Nxc3x97M pixels) can be addressed by N+M address lines.
Matrix addressing schemes for use with electrophoretic displays have been developed, for example by Dalisa (U.S. Pat. No. 4,203,106) and CopyTele (U.S. Pat. No. 4,655,987). However these require additional electrodes to be inserted between the cell walls (see FIG. 2). This is a complex manufacturing task.
According to the present invention there is provided an electrophoretic display having a matrix addressing structure comprising:
a first support structure on one side of the display, and on the inner surface of which are disposed a first set of parallel cathodes or anodes;
a second support structure on the other side of the display and arranged substantially parallel to the said first support structure, the second support structure having a surface facing the inner surface of the first support structure, said surface having disposed thereon a set of grid electrodes and a parallel set of anodes or cathodes respectively, the grid electrodes and anodes or cathodes being arranged substantially orthogonal to the set of cathodes or anodes respectively and being interspersed with one another.
Preferably, the grid electrodes and the anode or cathodes on the second support structure are interleaved with one another and they may be interdigitated with one another.
By adjusting the voltages on the gird electrodes and the anodes or cathodes and on the cathodes or anodes, the form of the potential gradient between the two parallel support structures can be controlled. For example, the field gradient can be linear or it can take the form of a potential well.
In the case that the particles in the display have a positive charge, then the grid and anode/cathode electrodes are positive with respect to the cathode/anode electrodes. In the case that the particles in the display have a negative charge, then the grid and anode/cathode electrodes are negative with respect to the cathode anode electrodes.
Advantageously, the cathodes or anodes on the first support structure form a set of column electrodes, the grid electrodes are connected in common, and the anodes or cathodes respectively on the second support structure form a set of row electrodes. The anodes or cathodes respectively on the second support structure may be connected in subsets of n electrodes, for example three electrodes per subset. Independently addressable row and column electrodes can thus be provided.
Preferably, the two support structures are separated by a distance of between 5 and 500 microns, and the space between them is filled with an insulating liquid in which a dye is dissolved. The liquid contains coloured particles and a particle charging agent.
Such a display may be integrated with other standard components and used to display variable price and other information, for example in a supermarket, or to form an electronic readout device for electronically stored information, such as an electronic book.
The invention also includes a method of operating an electrophoretic display as defined above, the method comprising the steps of:
(A) maintaining the grid electrodes and the anodes or cathodes at a relatively low voltage, while maintaining the cathodes or anodes at a relatively high voltage;
(B) placing the grid electrodes at a relatively high voltage;
(C) placing a voltage on selected ones of the cathodes or anodes intermediate that placed on the grid electrodes and the anodes or cathodes;
(D) raising the voltage on a selected one of the anodes or cathodes to the level of the voltage on the grid electrodes for a predetermined time and then lowering it again in order to adjust the display of individual pixels in the row corresponding to the selected cathode or anode; and
(E) repeating steps (C) and (D) as required in order to adjust the display of pixels in all rows as required.
In comparison with LCDs, switching can be much simpler as the display operates on DC voltages for both row and column electrodes, which can thus be driven by corresponding sets of relatively simple electronic switches.